1. Field of the Invention
The present invention relates to an improved optical filter and method of making the same for use in the measurement of combustion gas such as air-to-fuel ratio meters.
2. Description of Related Art
Optical filters and, particularly, optical filters utilized in measuring instruments for measuring the components in a gas by transmitting light through a chamber containing the gas, and determining the effect of the components in the gas on the transmission of the light, are known in the prior art. For example, in the analysis of the by-products resulting from the combustion of gasoline in a vehicle, gas analyzers such as an air-to-fuel ratio meter are utilized wherein light such as infrared light is transmitted through the combustion chamber, and a detector determines the intensity of the light that passes through the combustion gases. The combustion chamber is fitted with an optical filter assembly to permit the transmission of a desired bandwidth of radiation in order to determine the effects of the components in the combustion gas on that particular radiation.
Referring to FIG. 7, a schematic cross-sectional view of a prior art optical filter is disclosed, wherein a substrate 21 transmissible to infrared rays is formed, for example, of materials such as silicon (Si) and silica (sapphire). A band-pass surface layer 22 (hereinafter referred to as a BP surface) is capable of passing an appointed wavelength band of infrared radiation into the substrate. A second optical filter, such as a short-long cutting surface 23 (hereinafter referred to as a SLC surface), also provides a band-pass transmission capable of cutting the shorter spectrum wavelength band and the longer spectrum wavelength band of infrared rays to remove any noise components not necessary for the purpose of detecting the effect of the components in the combustion gas on the radiation. Generally, the BP surface 22 and the SLC surfaces 23 are formed of multilayer film depositions comprising, for example, germanium (Ge) and silicon monoxide (SiO), as known in the prior art.
Normally, the BP surface 22 is relatively thinly coated all over the surface of the substrate 21, and the SLC surface 23 is deposited to be considerably thicker than the BP surface 22. The SLC surface 23 has also been conventionally formed to be slightly smaller in dimension than the substrate 21, so that all the circumferential or peripheral portions of the substrate 21 will be exposed.
In the production of the conventional optical filter assembly, a masking plate 25, which is shown in an open configuration in FIG. 8, has a plurality of perforated portions 24 formed at spaced intervals corresponding to the desired SLC surface 23 that are to be deposited on the common substrate 26. The thinner BP surface 22 is deposited across the entire lower surface, as shown in FIG. 8. During the production, the mask plate 25 is mounted on the surface of the substrate slab 26, and two different kinds of material can then be vapor coated in a vacuum process to form the plurality of SLC surfaces 23 in a side by side arrangement on the substrate 26. Spaced between each of the SLC surfaces 23 are exposed portions of the substrate to provide cutting intervals 27. Subsequently in the production step, the substrate slab 26 and the coating BP surface 22 are cut along the cutting intervals 27 to form a plurality of optical filters.
It is necessary to form the SLC surfaces 23 in spaced individual portions on the slab substrate 26 and thereby provide the cutting intervals 27, because problems will occur if the SLC surfaces 23 extend over the entire slab substrate 26. A problem of tipping occurs during the cutting step so that an irregular side peripheral surface to the coating layer is provided. This irregular surface can provide interference problems with incident light. This problem is particularly acute with the relatively thick SLC surface, although the problem also occurs in a minor fashion with regard to the BP surface. Accordingly, the cutting intervals 27 are formed between the SLC surfaces during the production prior to the cutting step to eliminate this problem. Reference can be made to FIG. 7 for an example of the irregular edge 28 that could occur, and represents the tipping problem referred to above. If there is also a desire to eliminate any tipping on the BP surface 22, it is advantageous that the substrate 26 not be formed of an opaque material such as Si, and that the substrate rather be formed of a transparent material such as silica, to enable accurate alignment in the cutting procedure. Thus, if the substrate 26 is formed of a transparent material such as silica and sapphire, it would be possible to confirm the position of the BP surface portions 22 (not shown) and the SLC surface portions 23 during their formation. The BP surface portions 22 can then be equal to the SLC surfaces 23 in size and position, thereby eliminating the necessity of cutting the BP surface 22.
This solution, however, presents additional problems in that even if the BP surface portions 22 and the SLC surface portions 23 are formed equal in size to each other, there is still a possibility that they can be slightly shifted in spatial position, or could be slightly different in size, and thereby bring about an unmatched or nonoverlaid portion through which light can leak. In addition, when the substrate 26 must be formed of an opaque material such as Si, it is then additionally difficult to visually confirm the actual positions of the BP surface 22 and the SLC surfaces 23 so that problems associated, for example, with positional shifting between the respective BP surface 22 and SLC surfaces 23 are more likely to occur during production.
In applications where the area of the SLC surface 23 and the BP surfaces 22 are comparatively large, the leakage of light can be eliminated by utilizing only a central portion of the filter. Industry, however, is trying to make measuring instrumentation as small as possible commensurate with the reduced size of electronics. In an optical figure having a comparatively reduced area for both the BP surface 22 and the SLC surfaces 23, it is particularly difficult to remove any possibility of leakage resulting from irregularly formed and cut peripheral filter coating edges.
For example, in a was analyzer detector that is seeking to simultaneously measure three separate ingredients or components in a gas, at least four optical filters are required to be mounted on a vessel usually made of metal. One of the optical filters generally provides a standard reference, while the others are dedicated to each of the individual ingredients or components that are desired to be measured. If a leakage of light occurs in any of one optical filters, the interferential value is increased, and a highly accurate measurement of the three components cannot be achieved. Additionally, if there is a leakage of light, then the optical filters must be exchanged, and it is frequently difficult to replace such filters on a combustion chamber.
In order to improve measurement instruments for measuring the components in combustion gases by transmitting light through optical filter assemblies, it is highly desirable that the optical filter be manufactured so that there is no possibility of a leakage of light to interfere with the measurement calculations.
The prior art is still seeking to optimize the use of optical filter assemblies for measuring instrumentations.